Conventionally, a projection-type display unit is well known, which forms an optical image in accordance with a video signal on a light valve, and illuminates the optical image with light to magnify and project the optical image onto a screen with a projection lens so as to obtain a large screen video image. If three light valves are used corresponding to light beams of three primary colors of red, green, and blue, a projected image with high resolution and satisfactory color reproduction can be displayed.
A projection-type display unit using three light valves generally includes a light source, an illumination optical system for condensing light emitted from the light source onto light valves, the light valves, a color separating and synthesizing optical system for separating white light from the light source into light beams of three primary colors of red, green, and blue in an illumination optical path, and synthesizing the light beams of three primary colors output from the light valves into one light beam, and a projection lens for magnifying and projecting optical images formed on the light valves onto a screen.
Among the optical performance characteristics of a projected image displayed on the screen, resolution mainly is determined by the number of pixels of the light valves and the resolving power of the projection lens, light output mainly is determined by the total amount of light output from the light source, the condensing efficiency of the illumination optical system, the light utilization efficiency of the light valves, and the spectrum utilization efficiency of the color separating and synthesizing optical system, and color reproduction is determined by the spectral characteristics of the color separating and synthesizing optical system.
Among them, the color separating and synthesizing optical system, contributing to the light output and color reproduction, mostly has a configuration in which a blue reflecting dichroic mirror for reflecting blue light and transmitting red light and green light, and a red reflecting dichroic mirror for reflecting red light and transmitting green light and blue light are used to separate white light into light beams of three primary colors of red, green, and blue and synthesize the light beams of three primary colors (e.g., see JP4(1992)-372922A).
Hereinafter, a conventional color separating and synthesizing optical system will be described below, which uses, for example, a blue reflecting dichroic mirror configured so that the incident angle of light from air is 47.4°, and a red reflecting dichroic mirror interposed between two prism-shaped glass substrates in contact with each other and configured so that the incident angle of light from glass is 10°.
FIG. 10A is a graph showing a spectral transmittance of the blue reflecting dichroic mirror, and FIG. 10B is a graph showing a spectral transmittance of the red reflecting dichroic mirror. Both the dichroic mirrors have a configuration in which alternating periodic layers each including dielectric thin films as low-refractive-index layers and dielectric thin films as high-refractive-index layers are stacked on a glass substrate, and SiO2 with a refractive index of 1.46 is used for the low-refractive-index layers, and Nb2O5 with a refractive index of 2.3 is used for the high-refractive-index layers. The blue reflecting dichroic mirror has a wavelength exhibiting a transmittance of 50% (hereinafter referred to as a “half-value wavelength”), which is a wavelength band for separating blue light from green light, in the vicinity of 505 nm, and the red reflecting dichroic mirror has a half-value wavelength for separating red light from green light in the vicinity of 595 nm. It is understood that, in the red reflecting dichroic mirror, the transmittance curve in the vicinity of a half-value wavelength has a linear shape, whereas in the blue reflecting dichroic mirror, the transmittance curve in the vicinity of a half-value wavelength has a step shape in the vicinity of a transmittance of 50%.
The reason for this is that the incident angle of light with respect to the blue reflecting dichroic mirror is large. More specifically, as the incident angle is increased, the difference in half-value wavelength of a transmittance curve between S-polarized light and P-polarized light becomes larger as shown in FIG. 11, so that average light thereof has characteristics as shown in FIG. 10A. Herein, the S-polarized light refers to linearly polarized light whose plane of polarization is vertical to a plane including a normal line to a dichroic mirror surface and a light traveling direction, and the P-polarized light refers to linearly polarized light whose plane of polarization is vertical to the S-polarized light.
In the case of the blue reflecting dichroic mirror shown in FIG. 10A, the spectral transmittance curve in the vicinity of a half-value wavelength has a step shape, so that a part of a wavelength band on a blue side is transmitted on a green side, and a part of a wavelength band on the green side is reflected on the blue side, which degrades each color purity. Furthermore, if an attempt is made to enhance the color purity with this configuration, it is necessary to add, for example, a first color correction filter 61 for reflecting cyan light, and a second color correction filter 62 for reflecting yellow light for adjusting the chromaticity of white light whose chromaticity characteristics are changed due to the use of the first color correction filter 61, as shown in FIG. 12, which results in a large decrease in a spectrum utilization efficiency.
Thus, if a wavelength separating curve in the vicinity of a half-value wavelength also is linear in the blue reflecting dichroic mirror shown in FIG. 10A in the same way as in the red reflecting dichroic mirror shown in FIG. 10B, and the separation width between a reflection wavelength band and a transmission wavelength band can be decreased, the color reproduction and spectrum utilization efficiency can be satisfied.